1. Field of the Invention
The present invention relates to methods and apparatuses for data transmission, and more specifically, to methods for calculating and broadcasting cell-load ratio information, and methods for a cell selection and a handover initiated by a subscriber based on the cell-load ratio information and signal strength of base stations and apparatuses performing these methods.
2. Description of the Related Art
IEEE Standards 802.16-2005 (also known as IEEE802.16e and mobile WiMAX) “provide enhancements to IEEE Standards 802.16-2004 to support subscriber stations moving at vehicular speeds and thereby specify a combined system for fixed and mobile broadband wireless access.” In IEEE Standards 802.16-2005, “functions to support higher layer handover between base stations or sectors are specified.”
IEEE Standards 802.16-2005 however, do not disclose cell-load information in any base station (BS) broadcast message, such as DL-MAP (Downlink Access Definition) message, UL-MAP (Uplink Access Definition) message, DCD (Downlink Channel Descriptor) message, UCD (Uplink Channel Descriptor) message, MOB-NBR-ADV message (neighbor advertisement message), etc. In the contemporary wireless data transmission system constructed as IEEE Standards 802.16-2005, a contemporary mobile station (MS) does not consider the cell load of a target base station (BS) during the selection of a designated cell in the initialization stage or in the handover stage; and a serving BS does not consider the cell load of the neighboring BSs during performing a handover of the MS to one of those neighboring BSs. The handover process is a process in which an MS migrates from the air-interface provided by one BS to the air-interface provided by another BS.
The absence of cell-load information in IEEE Standards 802.16-2005 may introduce several problems, such as the call interruption during the handover process, the low throughput, the long delay and jitter, the high rate of call drop, and thus deteriorating the QoS (quality of service) of the transmission system.
Looking beyond contemporary IEEE Standard 802.16e, i.e., the mobile WiMAX technology, the IEEE is constructing IEEE Standards 802.16m for the gigabit WiMAX technology and IEEE Standards 802.16m may be possibly merged with LTE (long term evolution).
It is therefore necessary for standards IEEE802.16m to employ the cell-load information for data transmission in LTE, LTE+ (long term evolution plus), 4G (fourth generation) and 4G beyond systems in order to improve the quality of service of these systems.